A difficult obstacle associated with the exploration and production of oil and gas is management of significant ocean currents. These currents can produce vortex-induced vibration (VIV) and/or large deflections of tubulars associated with drilling and production. VIV can cause substantial fatigue damage to the tubular or cause suspension of drilling due to increased deflections. Deflections of both production and drilling tubulars can be alleviated using various types of fairings.
Fairings streamline the flow and thereby reduce the intensity of vortex shedding. With weaker vortices, both VIV and drag or deflection of the underlying tubular can be decreased.
For drilling risers the suppression device of choice is usually a fairing. This is because fairings offer both reduced VIV as well as reduced drag (whereas helical strakes only reduce VIV and can increase drag). A popular choice for drilling risers is the use of tail fairings which consist of a fairing tail and two straps. The straps only cover a fraction of the tubular and thus much of the riser is left directly exposed to the ocean. A challenge associated with tail fairings is that many risers have mux lines, pod hoses, or other auxiliary lines that run the length of the riser and can be exposed when tail fairings are used. This exposure can actually cause tripping of the boundary layers thereby significantly reducing the effectiveness of the tail fairing. In addition, it can be important to protect the auxiliary lines so that the fairing tail does not impact the auxiliary lines and cause damage to them.
Normally, when external auxiliary lines are present, full fairings are used that completely shroud the riser and mux lines. The problem with most full fairings are that they are slow and cumbersome to install, unlike tail fairings which are very fast and efficient to install. Finally, it is also important to have a full fairing that can be built quickly and at reduced cost.
Fairings are normally constrained axially by collars. These collars are clamped tight around the tubular so that the fairings may be free to rotate with changes in current direction and restricted from sliding axially past the collar. Various arrangements of collars and fairings are possible.
An issue associated with fairings is that they typically are made of plastic which requires a mold to form fairings of the correct shape. This requires fabrication of the mold prior to commencement of fairing fabrication, which adds for time and cost. Molding of fairings also takes considerable time and there is a cost associated with molding.
In general, fairings are significantly more expensive to fabricate than helical strakes, thereby giving them a disadvantage in the marketplace. They also take up considerable space during transportation which adds to their cost. While helical strakes are often split into halves or even thirds to increase the quantity that will fit into a shipping container, fairings cannot be split into halves or thirds because of they are not banded onto the tubular like helical strakes and thus need their full circumference for structural integrity.